1. Field of the Invention
The present invention relates to the production of silicon and more particularly to an improved system for the production of solar grade silicon by the thermal decomposition of silane.
2. Description of the Prior Art
Self sufficiency in energy is a stated national goal. Most of the proposed means to achieve this goal are either environmentally unacceptable or are not feasible, especially those not depending on fossil fuel sources. Of the available alternatives, solar energy is the most abundant, inexhaustable single resource available. However, capturing and utilizing solar energy is not simple. Methods are being sought to convert solar energy to a concentrated, storable form of energy. A known method, photosynthesis, converts somewhat less than 1% of the suns energy at the earths surface to a solid fuel, i.e., plant materials, which when accumulated and transformed over geologic ages yielded fossil fuels. Current rates of use of these fossil fuels, and the particular geographic distribution and political control of major petroleum resources pose problems for nations that are net petroleum consumers. An alternate method yielding a simpler fuel, at a higher conversion, has long been desired.
One method of converting solar energy to a usable form being prominently considered is the development of large arrays of photovoltaic solar cells, especially in the sunbelt areas such as the southwestern and western regions of the United States. The most promising candidate for the solar cell is a doped silicon sheet material and silicon is one of the most plentiful elements in the earth's crust.
The most abundant source of silicon is silica sand. Metallurgical silicon can be made cheaply from sand in an arc furnace by reaction with carbon to produce silicon and CO.sub.2. This material, however, is far too impure for use in solar cells. To purify this material, the silicon can be converted to a gaseous product where distillation and adsorption can be used for purification. For example, metallurgical silicon will react with chlorine to produce SiCl.sub.4 gas. This gas can be converted to silicon using a reductant such as zinc, as studied by Battelle Memorial Institute, or to silane gas, SiH.sub.4, by a redistribution reaction with hydrogen. This highly pure silane gas can then be thermally decomposed to silicon and hydrogen.
The thermal decomposition of highly pure silane gas to produce silicon is preferred over the formation of silicon from SiCl.sub.4. The silane thermal decomposition reaction is preferred due to its simplicity, potential purity (no zinc or other reactant), and the relatively mild conditions of reaction.
A major problem present in producing high purity silicon by thermally decomposing chlorosilane gas is that known processes include inherently wasteful or inefficient energy consumption steps. For example, the Siemens process for the production of high purity silicon on heated rod surfaces has inherent massive heat losses.
The current energy crisis has especially focussed attention on the major deficiencies of these conventional chlorosilane thermal decomposition processes, where huge expenditures of energy are required per pound of product. The need for new processes to produce high purity silicon in great volume, high efficiency or yield and at very low overall energy expenditure levels is expected to become more critical as the world turns to more widespread use of silicon solar energy collection systems.
An approach which has been taken to overcome the inherent energy waste present in prior processes utilizes a fluidized bed reactor (FBR) for thermally decomposing silane. Fluidized bed reactors are well known for their excellent heat and mass transfer characteristics.
Examples of fluidized bed reactor utilization to thermally decompose silane to form silicon are disclosed in U.S. Pat. Nos. 3,012,861 and 3,012,862.
These patents disclose use of a fluidized bed of ultra pure silicon seed particles on which silicon is deposited during thermal decomposition of silane, chlorosilanes or other halo silanes within the fluidized bed. The silanes are introduced along with preheated fluidizing gas, such as hydrogen or helium into the fluidized bed reactor. Heat for the silane decomposition reaction is supplied by conventional external heating elements in conjunction with the preheated gas. The fluidizing gas containing the silanes maintains the bed of pure silicon seed particles in a fluidized state. The silanes present in the fluidizing gas thermally decompose within the fluidized bed reactor and are deposited on the seed crystals as opposed to the reactor side walls or other surfaces.
As the silicon seed particles grow within the fluidized bed, the heavier particles migrate to the lower portion of the fluidized bed where they are removed for further processing as a highly pure silicon product. This method provides a relatively energy effecient process for producing the high purity silicon required in solar arrays.
The continuous removal of silicon seed particles after they have grown to the desired size results in the depletion of the fluidized bed. In order to maintain a continuous bed of silicon seed particles within the reactor, it is necessary to continually introduce additional silicon seed particles into the fluidized bed to replace those removed as part of the ultra pure silicon product.
This necessity of providing a continuous supply of ultra pure silicon seed particles to the fluidized bed reactor presents a serious problem to which no adequate solution has yet been found. For example, methods utilized for producing silicon seed bed particles in the silicon fluidized bed processes being developed recently involve one of two techniques or a combination thereof. One of these processes involves crushing, classifying and cleaning ultra pure silicon into appropriately sized silicon seed particles. A hammer mill as well as jaw, cone or roller crushers are utilized to reduce the bulk silicon to a specific particle size distribution suitable for use as seed particles. The crushing and classifying process is not only expensive and time consuming but also presents severe contamination problems. In addition the crushing produces a highly acicular seed particle which presents an undesirable surface for efficient silicon deposition. Procedures to process the acicular seed particles into a more uniform shape involve dry tumbling or wet tumbling in the presence of water or methanol. Possiblities for contamination and long tumbling times render this type of procedure undesirable.
The other process for producing silicon seed particles is not plagued with the above mentioned problems inherent in crushing and grinding processes, but it also presents significant problems of its own. This process involves the recycling of appropriately sized seed particles which are generated in the fluidized bed reactor and removed along with the larger silicon product particles or entrained overhead in fluidizing gas exiting the reactor. In the reactor, a certain amount of silicon crystallizes spontaneously to form so-called homogeneous particles, while the majority of the silicon formed through thermal decomposition undergoes heterogeneous crystallization on the surface of the seed silicon particles. These homogeneous silicon particles form the bulk of the silicon particles recycled back into the reactor as seed particles.
In general, there is not a sufficient amount of appropriately sized homogeneous particles produced during normal reactor operations to supply the entire demand for new seed particles in the fluidized bed. This requires that the recycled stream of homogeneous particles be supplemented by seed particles produced by crushing or grinding. In addition, and more importantly, these homogeneously formed silicon particles tend to be amorphorous or not-dense in nature and present a very undesirable surface for elemental silicon deposition.
It is apparent that there is a present need for a method of providing a continuous supply of appropriately sized suitable silicon seed particles without the problems inherent in the above two discussed processes. Until a convenient, economical and suitably pure source of silicon seed particles is found, the fluidized bed process for producing ultra pure silicon will be less than optimally desirable.